1. Field of the Invention
This invention relates generally to a corona discharge ignition system, and more particularly to controlling energy supplied to the system.
2. Related Art
Corona discharge ignition systems provide an alternating voltage and current, reversing high and low potential electrodes in rapid succession which enhances the formation of corona discharge and minimizes the opportunity for arc formation. The system includes a corona igniter with a central electrode charged to a high radio frequency voltage potential and creating a strong radio frequency electric field in a combustion chamber. The electric field causes a portion of a mixture of fuel and air in the combustion chamber to ionize and begin dielectric breakdown, facilitating combustion of the fuel-air mixture, which is referred to as an ignition event. The electric field is preferably controlled so that the fuel-air mixture maintains dielectric properties and corona discharge occurs, also referred to as a non-thermal plasma. The ionized portion of the fuel-air mixture forms a flame front which then becomes self-sustaining and combusts the remaining portion of the fuel-air mixture. Preferably, the electric field is controlled so that the fuel-air mixture does not lose all dielectric properties, which would create thermal plasma and an electric arc between the electrode and grounded cylinder walls, piston, metal shell, or other portion of the igniter. An example of a corona discharge ignition system is disclosed in U.S. Pat. No. 6,883,507 to Freen.
In addition, the corona discharge ignition system is preferably controlled so that energy is provided to the corona igniter at a drive frequency equal or close to the resonant frequency of the corona igniter. This provides a voltage amplification leading to robust corona discharge in the combustion chamber. Detecting the resonant frequency of the corona igniter is necessary in order to achieve this high level of control. However, accurate detection of the resonant frequency it is difficult to achieve, especially at a wide range of frequencies. Changes in the resonant frequency during operation, for example due to arcing events, also make it difficult to accurately detect the resonant frequency.